It is expected that network densification will play a major role in beyond 4th generation (4G) networks in order to handle the increasing data traffic demands. In this dense scenario, the measurement of downlink reference signals by the user node (UNd) from a multiplicity of access nodes (ANds) and the transmission of measurement reports back to the network side has been identified in the literature as a suboptimal solution. Some of the reasons are high power consumption having to measure a large number of ANds (due to high expected ANd density) and high signalling load sending back these measurements to the network. In addition, expected smaller cell ranges would impose very stringent requirements on when measuring report feedback should be sent back before they become obsolete. This is particularly true for radio access mobility or ANd (re)selection where measurements are required on a timely manner. Therefore, it is proposed that the UNds transmit uplink (UL) reference signals (henceforth uplink beacons) which are then measured at the network side, possibly by more than one ANd. Reception at multiple ANds is possible provided ANds have common information on the allocated resources for such transmissions, or ANds are able to detect UNd transmissions cross-correlating with known signature sequences transmitted by the UNd. An additional benefit of transmitting uplink beacons is that the use of conventional paging for tracking the UNd can be avoided. Paging consumes a significant amount of bandwidth for each event arrival for a UNd and it is not scalable with a large number of mobile users as assumed in Ultra-Dense Networks (UDNs). The absence of paging also reduces the energy consumption at the UNd.
3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) operation is heavily based on the measurement of downlink reference signals by the user equipment (UE) and reporting back those measurements to trigger cell reselection (i.e. handover) if necessary. As for the uplink, LTE implements so-called Sounding Reference Signals (SRS) which are used by the eNB to figure out the channel quality in the uplink and hence have better information on where to allocate resources for a particular user. Allocation and configuration of SRS is signalled to the user equipment from the network side.
If uplink reference signals are to be used for mobility purposes and/or location update purposes, the applicability of SRS in LTE seems limited. Indeed, SRS periodicity transmission in LTE is restricted to the set {2, 5, 10, 20, 40, 80, 160, 320} ms, and any change of the periodicity has to be done via Radio Resource Control (RRC) messages. Managing mobility in dense small cell networks will require more flexibility in allocating uplink beacon resources, especially in cases where UNd density is high. Flexibility both in allowing more periodicities than those allowed by LTE (i.e. higher granularity), and periodicities below those allowed by LTE (i.e. more frequent beacons).
Thus, in order to enable implementation of ultra-dense networks, new solutions are required for making intra control node handover of UNds.